Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touch object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
To an increasing extent, touch systems are designed to be able to detect two or more touches simultaneously, this capability often being referred to as “multi-touch” in the art.
There are numerous known techniques for providing multi-touch sensitivity, e.g. by using cameras to capture light scattered off the point(s) of touch on a touch panel, or by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a touch panel.
WO2011/028169 and WO2011/049512 disclose multi-touch systems that are based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel to propagate inside the panel by total internal reflection (TIR). When an object comes into contact with a touch surface of the panel, the propagating light is attenuated at the point of touch. The transmitted light is measured at a plurality of outcoupling points by one or more light sensors. The signals from the light sensors are processed for input into an image reconstruction algorithm that generates a 2D representation of interaction across the touch surface. This enables repeated determination of current position/size/shape of touches in the 2D representation while one or more users interact with the touch surface.
The touches need to be detected in the presence of interferences, such as measurement noise, ambient light, contamination on the touch surface (e.g. fingerprints and other types of smear), vibrations, etc. These interferences may vary not only over time but also across the touch surface, making it difficult to properly detect the touches on the touch surface at all times. Furthermore, the degree of interaction between a touching object and the touch surface may vary both over time and between different objects. For example, the interaction may depend on if an object is tapped, dragged or held in a fixed position onto the touch surface. Different objects may yield different degree of interaction, e.g. the degree of interaction may vary between fingers of a user, and even more so between the fingers of different users.
The combination of several touches, complex gestures as well as temporal and spatial variations of interaction and interferences will make the identification of touches a demanding task. The user experience may be hampered if, e.g., an ongoing gesture on a touch screen is interrupted by the system failing to detect certain touches during the gesture.
The above-mentioned WO2011/028169 and WO2011/049512 propose different techniques to suppress the influence of contamination, e.g. by compensating the 2D representation by an intermittently updated 2D background status that estimates the interaction caused by contamination, or by normalizing the measurement signals by reference values that are intermittently updated to represent the influence of contamination. To the extent that such a suppression is incomplete, or other types of interferences are present, the 2D representation may still contain false peaks that are mistaken for touching objects.
The prior art also comprises WO2010/006883 which discloses a technique for touch determination in an optical multi-touch system, where attenuated light paths are identified and processed for determining a set of candidate touches, which may include true touches as well as ghost touches. The candidate positions are then validated based the shape and/or area of each candidate touch, allowing the ghost touches to be identified and eliminated from further processing.
The present Applicant has identified a need for an improved technique of tracking objects on the touch surface in the presence of interferences.